1. Field of the Invention
The invention relates to a high-transparency silicone composition having improved mechanical properties and to use thereof in optical components.
2. Description of the Related Art
Casting compounds for optical semiconductor components such as LEDs (light emitting diodes) or materials for compression molding or injection molding, for the production, for example, of lenses for secondary optical systems, are required to protect the components from ambient mechanical and chemical influences, to be thermally robust, to exhibit high transparency, and to ensure a high level of outcoupling of light. Common materials for the casting of LEDs in this context are, for example, epoxy compounds or silicone compounds.
There has been a preference for silicone over epoxy casting compounds with respect to thermal stability.
For use as primary or secondary optical systems in optical elements, the silicone systems are required to exhibit high optical transparency in the visible range and to some extent also in the UV range (UV-Vis) of the electromagnetic spectrum. With crosslinked polydiorganosiloxanes, high transmission levels are achieved at wavelengths down to below 300 nm. A drawback of these systems is that the hardnesses are limited to the lower Shore A range and the mechanical robustness is very low. When silicone resins are used, significantly higher hardnesses can be achieved (upper Shore A through into the shore D range), with similarly good transmission levels.
EP 1424363 B1 describes compositions comprising alkenyl-functional silicone resins in combination with SiH components, the vulcanizates thereof having hardnesses in the Shore D range.
Silicone systems with alkyl substituents have refractive indices nD25 of around 1.41. By using aryl substituents such as phenyl it is possible to extend the refractive index to nD25>1.50. The use of casting compounds with such extended refractive index in optical semiconductor components improves light outcoupling and so leads to greater efficiency on the part of the components. U.S. Pat. No. 7,282,270 B2 describes corresponding compositions comprising alkenyl- and aryl-functional silicone resins in combination with Si—H components, featuring high refractive indices. Through the use of silicone resin formulations it is certainly possible to achieve vulcanizates having hardnesses in the Shore D range. Nevertheless, other mechanical properties, such as the elongation at break, for example, remain very low. This is a disadvantage in casting compounds or components in secondary optical systems, since the high thermal stresses cause increased cracking in the cured material if the elongation at break is low.
The mechanical properties of silicones are generally improved by adding reinforcing fillers having specific surface areas of between 50 and 400 m2/g. Examples of active reinforcing fillers include finely divided fumed or precipitated silicas or else other pyrogenic metal oxides. Vulcanizates made from such silicone rubbers reinforced with fumed silica, for example, are nevertheless no longer of high transparency, owing to optical scattering effects, even if the particles of filler are smaller than the wavelength of visible light.
To obtain optically transparent mixtures, it is necessary for the refractive indices of filler and polymer to be adapted to one another.
EP 0644914 B1 describes a “Process for the production of optically homogeneous, highly transparent or light-scattering polymeric shaped articles or of embedding compositions” composed of organic matrix materials and inorganic fillers such as metal oxides, the refractive index of the filler particles being adapted to the refractive index of the organic matrix. Thus, for example, SiO2/TiO2 mixed oxide particles having a refractive index of 1.52 are produced, corresponding to the refractive index of an epoxy resin system used for embedding optical components, thus allowing high-transparency mixtures.
In contrast to this, US 2012/0235190 A1, “Encapsulant with index matched thixotropic agent”, for example, describes how fumed silica (SiO2) is often used as a thixotropic additive. As a result of the differences in refractive index n between silicone polymer used (presently n=1.51) and fumed silica (n=1.46), however, the light of the LED can be scattered, possibly leading to hazing of the casting compound and to a reduction in luminous efficiency. Instead of fumed silica, therefore, composite additives such as aluminosilicates, for example, are used preferentially as thixotropic additives, having refractive indices which differ only little from those of the polymers used. The specification gives no pointers to modifying the mechanical properties through use of such thixotropic additives.
In the poster presentation PO-173 at the 17th International Symposium for Silicone Chemistry [ISOS XVII BERLIN 2014, ISBN 978-3-936028-85-0], the author discloses the refractive indices of fillers based on pyrogenic metal mixed oxides (trade name AEROXIDE®, from Evonik) such as SiO2/Al2O3, for example, as a function of the mixing ratio. Given a suitable mixing ratio, the refractive index of the metal mixed oxide can correspond to the refractive index of phenyl-containing polyorganosiloxanes that are often used in optical applications. These mixtures adapted in terms of the refractive index of filler and polymer exhibit greater transparency than mixtures in which filler and polymer have different refractive indices. The author, moreover, indicates that adaptation of the refractive index of fumed silica to the refractive index of the new methyl-phenyl-polysiloxane matrix is not possible. Consequently, such systems have poor transparency and thus a lower effectiveness as a result of the increased light scattering. In his paper “AEROXIDE® Fumed Metal Oxides—Fillers for Optical Applications” on May 21, 2014 at the “2014 International Silicone Conference” in Akron, Ohio, USA, the presenter, Simon Nordschild, Evonik Industries AG, disclosed mechanical properties for the silicones filled with these mixed oxides. It emerged, however, that they are substantially poorer than the mechanical properties of the systems filled with fumed silica.
EP 2336230 A1 describes the use of cristobalite with refractive index n=1.53 as a thermal conductivity-improving filler in silicone compositions having a refractive index of, e.g., n=1.51. The filler differs in refractive index by not more than +/−0.03 from the refractive index of the vulcanizate of the polymer composition. As a result of the similarity in refractive index, it ought to be possible to produce transparent vulcanizates. Cristobalite, however, does not improve the mechanical properties.
There is therefore a need for silicone compositions having a refractive index higher than that of standard addition-crosslinking polymethylsiloxane systems (which have a refractive index of nD25=1.41) which at the same time exhibit improved mechanical properties as compared with existing systems. The requirement is for greater hardnesses in conjunction with improved elongation at break and consistently high transparency.